1. Field of the Invention
The present invention relates to a light-emitting semiconductor device that emits blue light and uses a group III nitrogen compound.
2. Description of the Related Art
It has been known that an aluminum gallium indium nitride (AlGaInN) compound semiconductor may be used to obtain a light-emitting diode (LED) which emits blue light. Such a semiconductor device is useful because it has a high luminous efficiency resulting from direct electron transition and because it emits blue light, which is one of the three primary colors.
By irradiating an electron beam into an i-layer, to which magnesium (Mg) is doped and heat treatment is carried out, the formed i-layer serves as a p-type layer of the AlGaInN semiconductor device. As a result, an LED with a double hetero p-n junction structure including an aluminum gallium nitride (AlGaN) p-layer, a zinc (Zn) doped indium gallium nitride (InGaN) emission layer and an AlGaN n-layer, becomes useful instead of a conventional LED of metal insulator semiconductor (MIS) structure which includes an n-layer and a semi-insulating i-layer.
The conventional LED with a double hetero p-n junction structure is doped with Zn as an emission center. The luminous intensity of this type of LED represents a fair improvement. However, further improvement in luminous efficiency is desired.
The emission mechanism of an LED with an emission layer doped with only Zn, or only an acceptor impurity, as the emission center is electron transition between conduction band and acceptor energy levels. However, a large difference in their energy levels makes recombination of electrons through deep levels dominant, and such deep level recombination does not contribute to emission. This results in lower luminous intensity. Further, the wavelength of light from the conventional LED is about 380 to 440 nm, shorter than that of pure blue light.
Further, an emission layer doped with Zn in its emission center exhibits semi-insulative characteristics. Its emission mechanism is explained by recombination of an electron through acceptor level injected from an n-layer and a hole injected from a p-layer. However, the diffusion length of the hole is shorter than that of the electron. This results in a high ratio of holes disappearing in a non-emission process before recombination of the hole and electron occurs in the emission layer. This phenomenon impedes the desired obtention of a higher luminous intensity.